Stainless steel resistant to nitric acid corrosion



United States Patent 3,129,120 STAXNLESS STEEL RESISTANT T0 NITRIC ACID C(BRROSION Raymond Smith, Pittsburgh, Pa., and Charles 0. Tan,

Cincinnati, Dhio, assignors to United States Steel Corporation, a corporation of New .lersey No Drawing. Continuation of application Ser. No. 779,571, Dec. 11, 1958. This application Feb. 5, 1962, Ser. No. 171,272

2 Claims. (Cl. 143-38) This invention relates to molybdenum-containing chromium-nickel austenitic steels possessing a high degree of resistance to nitric acid corrosion.

It is generally recognized that molybdenum-containing chromium-nickel austenitic stainless steels such as AISI type 31'6-L has relatively poor corrosion resistance to boiling nitric acid when tested after having been exposed to a sensitizing heat treatment. However, the use of the type 316L is desirable since it has excellent corrosion resistance to many acid mixtures and thus renders any container made from 316-L more versatile. A penetration rate of less than .002" per month is an indication of satisfactory resistance to nitric acid corrosion.

It is accordingly an object of the present invention to provide a molybdenum-containing austenitic chromiumnickel steel of the so-called 18-8 type such as AlSI 316L having a penetration rate of less than .00 per month in boiling 65% nitric acid after sensitizing for 2 hours at 1250 F. and air cooling.

Previous work has rather definitely shown that maintaining the carbon content of austenitic-chromium-nickel steels of the 188 type below .03% will result in such steels having an adequate resistance to nitric acid corrosion as evidenced by a penetration of less than 0.002" per month in boiling nitric acid. However, it is frequently desired to use molybdenum-containing steel such as AISI 316L which has a nominal composition of .03% maximum carbon, 16.00 to 18.00% chromium, 10.00 to 14.00% nickel and 2.00 to 3.00% molybdenum. As is be completely austenitic. The composition may then be set forth as follows:

Carbon .03% max. Manganese 2.00% max. Molybdenum 2.00 to 3.10%. Silicon .60% max. N ckel 14.30% min. Chronnum 17.00% min.

with the balance iron and residual impurities. The grade and economic consideration will ordinarily provide a 16% upper limit for nickel and a 20% upper limit for chromium although more can be used if desired. Nitrogen up to about .25 is not detrimental to the properties.

The preferred range for the steel is as follows:

Carbon .03% max. Manganese 1.50% max. Phosphorus LAP.

Sulphur LAP. Molybdenum 2.00 to 3.10%. Silicon LAP.

Nickel 14.5% min. Chromium 17.00 to 18.5%.

1 Low as possible.

Within such limits a completely austenitic steel may be obtained if the austenite formers manganese, carbon and nickel are properly adjusted with respect to the ferrite formers molybdenum, silicon and chromium and the steel is severely hot worked while at a temperature in the range of 1200 to 2000 F. to reduce cross-sectional area at least 60% and then air cooling. The steel must not thereafter be reheated above 2000 F. unless it is again similarly substantially reduced in cross section while within such temperature range. As is seen from the steels listed in the following Table I, it is not suificient that the composition be austenitic to get the desired resistance but also that the maximum silicon and minimum nickel limits are observed.

Table I Corrosion Steel No. 0 M11 P S S1 N1 Or M0 N Rate Ins. Structure Penetra- Present tion/Month .66 14. 09 18. 49 2. 00 00459 A .63 13. 06 19. 27 3.10 03507 A+F .70 18.11 19.32 3. 10 00326 A+F 96 20.16 18. 60 2. 95 00395 A 41 12. 40 19.00 2. 00 01630 A+F 26 14. 12 16. 47 2. 00272 A-l-F 41 14. 32 18. 56 2. 60 00069 A 43 14. 94 18. 41 2. 55 00515 A+F .56 12. 66 17. 71 2. 55 22 00234 A+F 78 15. 49 19.51 3.00 23 00249 A-l-F .57 16. 95 20. 25 3.05 00057 A .72 14. 39 19.12 3.00 00401 A-I-F 58 15. 72 19.13 2.85 00184 A 51 15.80 18. 99 3.05 00067 A .26 15.05 16. 62 3.10 .0209 A-l-F 25 15.12 15.95 2. 60 00575 A-l-F 60 15.89 19.14 3.05 23 00082 A All samples annealed /2 hr.'195 0 F., W.Q. +2 hrs.

evident from certain examples hereinafter given in Table I, such steels have a quite poor resistance to corrosion by nitric acid. We have found that adequate resistance to nitric acid corrosion can be obtained in such steels by limiting the silicon content to .60% maximum, raising the nickel content to 14.3% minimum and maintaining the chromium content above about 17% and provided that within such limits the composition is adjusted to -1250 F., A.C. prior to testing.

This application is a continuation of our copending application Serial No. 779,571, filed December 11, 1958, abandoned, which was a continuation-impart of our application Serial No. 616,947, filed October 19, 1956, now abandoned.

While we have shown and described several specific embodirnents of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. Molybdenum-containing hot-worked, chromiumnickel, completely austenitic steel consisting essentially of Carbon Up to .O3%. Manganese Up to 2.00%. Silicon Up to .60%. Molybdenum 2.00 to 3.10%. Nickel 14.30 to 16.00%. Chromium 17.00 to 20.00%.

with the balance iron and residual impurities, said steel having been reduced in cross-sectional area at least 60% while in the temperature range of 1200 to 2000 F. and not thereafter heated above 2000 F., the composition of said steel and said hot Work combining to produce a delta ferrite free structure and resistance to nitric acid corrosion characterized by a penetration rate of less than .002 inch per month in boiling 65% nitric acid after sensitizing for 2 hours at 1250 F. and air cooling.

2. The method of producing molybdenum containing hot-worked, chromium-nickel, completely austenitic stainless steel articles characterized by a penetration rate less than .002 inch per month in boiling 65% nitric acid after sensitizing for 2 hours at 1250 F. and air cooling comprising forming a heat of steel consisting essentially of Carbon Up to .O3%. Manganese Up to 1.50%. Silicon 60% max. Molybdenum 2.00 to 3.10%. Nickel 14.30 to 16.00%. Chromium 17.00 to 20.00%.

balance iron and residual amounts of other elements and hot working said steel to reduce the cross-sectional area at least 60% while in the temperature range of 1200 to 2000 F. and thereafter maintaining the temperature of said steel below 2000 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,118,683 Newell et al. May 24, 1938 2,544,336 Linnert Mar. 6, 1951 2,562,854 Binder July 31, 1951 2,801,916 Harris et a1. Aug. 6, 1957 

1. MOLYBDENUM-CONTAINING HOT-WORKED, CHROMIUMNICKEL, COMPLETELY AUSTENITIC STEEL CONSISTING ESSENTIALLY OF 